In a flying spot telecine the film is scanned by light from a cathode ray tube. This cathode ray tube produces the raster or scanning light spot on a layer of phosphor. There often occur very small blemishes in this phosphor caused, for example, by foreign particles, missing phosphor, or separation of the phosphor layer. These blemishes result in degraded performance of the telecine by causing black spots to appear on the television picture (these will appear white when negative film is being used).
Various means have been used to minimize this difficulty. The first step is selection of the cathode ray tubes after their manufacture, which is a very expensive solution. Another technique, using a burn correction system, is to pick up the light directly from the phosphor (before the film) and to use this signal to increase an amplifier gain during the time at which the blemish is scanned. This technique is not satisfactory as it is quite difficult to match precisely the signals produced by the blemish which results in some residual visible blemish. Dirt in the path of the blemish detector can cause additional problems. It has also been suggested in GB-A-2229885 to map the locations of these blemishes and to cause the signal from those locations to be replaced with nearby video information. This also leaves visible defects in the picture area.